[FIX] website_sale: prevent update of cart qty in edit mode

/!\ Should not be forward-ported in *12.0* /!\

Because of the mechanism to bind change / click event in the 10.0
implementation, when we are in edit mode on a product page, the event
handlers that are in charge of updating the cart quantity can still be
triggered.

After this commit, click / change event while updating cart qty will
not be effective in edit mode.

task-1825889
Closes https://github.com/odoo/odoo/pull/24211

closes odoo/odoo#29224